Monitoring of pipes and canalisations in the oil and gas industry requires a large number of measurements to be performed in order to control or prevent the development of scale and corrosion or for managing and monitoring a fluid flow when the fluid has to be transported a long distance in a pipeline. The composition and the flow rates of individual components of a mixture of oil and water and possibly gas fluid flowing in a pipe need to be monitored. These measurements are needed to determine the constituents present in the oil well and also to help control and maximise oil extraction.
Conventional techniques for measuring composition and flow rates of individual components require separate measurement of the flow of each of the fluid components. Alternatively, multiphase meters can be used to measure the composition and the flow rates without prior separation. These meters measure the flow speed and the relative fractions of the oil, water and gas (it may also need the temperature, pressure, density of the oil and gas, and the water salinity as input parameters for compensational purposes). The space available for a multiphase meter in an off shore Christmas tree production system is limited. Therefore, there is a need for a compact multiphase meter.
Corrosion monitoring is another significant problem, particularly in the oil and gas industry. The aggressive influence of acids, alkaline solutions and gases cause corrosion in metals. Corrosion in plastics is caused due to the capture of foreign particles, UV light and heat. Both corrosion mechanisms can be monitored, inspected and tested by optimised sensor probes that operate at microwave or high frequency part of the electromagnetic spectrum.
The most widely used techniques for corrosion detection and monitoring in oil and gas pipelines are the Electrical Resistance (ER) monitoring and weight loss coupons. These methods detect metal loss. They fail to detect deterioration in paint or protective coating materials and the conditions responsible for the onset of corrosion. Non-Destructive Testing (NDT) techniques such as ultrasonics, radiography, thermography and eddy current measurement techniques are not sensitive enough for corrosion prognostics. Furthermore, paint, primer and corrosion products are typically dielectric (insulating) materials. Therefore, these methods are unsuitable for detecting and evaluating properties (i.e. presence and thickness) of corrosion layers under thin layers of paint and primer.
Corrosion of metal is a complex problem and its effect on commercial and industrial equipment is immense for the safety and integrity of a large array of assets. A better predictability of corrosion growth under insulation through the early detection of corrosion is needed. Recent results in near-field microwave non-destructive inspection techniques for detecting corrosion under paint and primer in aluminium panels are indicative of the potential advantages of using microwave signals. For example, U.S. Pat. No. 7,190,177 describes a microwave sensor for sensing rust under paint and composite. The sensor can be used for imaging the corrosion of materials depending on a measurement of phase shift of a reflected signal. The sensor can also determine the level of the bulk material from the propagation time of the pulse. However, the sensor cannot detect dielectric or material properties, and so cannot detect changes in these properties.
To simulate reservoir oil, measurements of properties of the formation rock such as porosity, permeability and fluid saturation are needed. Until recently, core samples were the only source of permeability. In addition, data collected from the reservoirs can be sparse and expensive to obtain. Nuclear Magnetic Resonance (NMR) data can be a valuable tool for collecting the permeability-porosity data. For example, U.S. Pat. No. 4,785,245 describes a NMR well logging tool used by the oil industry to determine, in situ, the porosity and permeability of fluid-rocks. Particularly for permeability determination, NMR is better than other well logging methods because the NMR signal relaxation times (T1 or T2) can be used to provide information about pore size distributions. NMR also provides a measure of the total hydrogen based in the rocks. Other studies show that another magnetic resonance technique—Electron Paramagnetic Resonance (EPR)—produces a detectable signal from organic-free radicals in crude oils, but not from water or from gas. The amplitudes of these EPR signals are proportional to the amount of oil inside the rock and should, therefore, directly measure the oil fraction in fluid-rocks or the oil fraction of the crude oil mixture. When used with NMR, this method can thus allow the detection of the components of water and oil in the rocks (and possibly the gas) separately.
EPR spectrometers in general detect the concentration and composition of free radicals in a sample. The sample is usually loaded into a high-frequency resonant cavity in a slowly varying uniform magnetic field. Unpaired electrons irradiated with microwave radiation at a fixed frequency undergo resonant transitions between the spin-up and spin down state at a characteristic magnetic field. The energy difference between these two energy levels is called the Zeeman splitting. For an electron in free space, the Zeeman splitting is equal to hv=gβH where v is the excitation frequency, H is the applied magnetic field, β is the Bohr magneton, h is Planck's constant, and g is a factor that depends on the molecule.
Most EPR measurements are made with microwaves in the 9000-10000 MHz (9-10 GHz) region with magnetic field intensities corresponding to about 3500 Gauss (0.35 T). For example, for the field of 3350 Gauss, electron spin resonance occurs near 9400 MHz (EPR) for an electron compared to only about 14.3 MHz (NMR) for nuclear magnetic resonance. Many EPR spectroscopy systems have difficulty with automatic frequency control (AFC) locking to a low Q resonator. This difficulty is more commonly experienced at low powers of less than −70 dbm. Difficulty in obtaining an AFC lock may cause frequency drift, error voltage, dispersion and noise. A higher Q system makes it easier to obtain a frequency lock without GaAs FET amplifications. Therefore, there is a need for an EPR probe having a high Q.